The invention relates to a radar system for the automatic tracking of targets, in particular targets which are relatively low altitude.
With a target at a designated range and azimuth, but still unknown elevation, such a radar system is aligned with the indicated range and azimuth during acquisition, whereupon the tracking antenna performs a motion in elevation. At the moment the tracking antenna acquires the target, the three-dimensional position (range, azimuth and elevation) of the target is known, so that the actual tracking phase of the radar system can be started. Since the target may change its position rapidly during the acquisition phase and, hence, the range and the azimuth may be subject to change during this phase, it is advisable to select a sufficiently wide range gate and beam width to prevent loss of the target during the elevation scan of the tracking antenna. Moreover, a not too narrow beam is of importance when an operational radar system is moved clear over a hilly terrain, while processing tilted positions as efficiently as possible.
Once the radar system has detected the target and has thus assumed the tracking phase, electronic readjustment of the range gate for a smaller value is a simple matter to obtain a better signal/noise ratio; however, it is not possible to reduce the beam width, as the antenna size is a fixed parameter. It is therefore necessary to continue tracking of the target with a too wide a beam width; this does not usually present any insuperable problems.
Should the target however retain a relatively low altitude, in particular with a relatively large target range, there is the problem that, due to reflection of a part of the echo signals against the earth surface, the tracking antenna just because of its excessively wide beam receives, in addition to the echo energy derived directly from the target being tracked, echo energy derived from the target but reflected by the earth surface. The result is that the reflected radiation pattern will be disturbed, and may impede the accurate tracking of the target. In other words, the advantage gained with a rather wide antenna beam in the acquisition phase changes into a disadvantage in the tracking phase, in particular with the tracking of a low-flying target at short range when it is desired to have a reasonably narrow antenna beam. Since larger dimensions of an antenna result in a narrower antenna beam, a solution for the above conflicting requirements is obtainable with a phased-array antenna according to the state of the art: in the acquisition phase of a given target the more centrally located antenna elements are utilised through a suitably selected amplitude and phase control of these elements, resulting in a rather wide antenna beam. On the other hand, in the tracking phase in respect of low flying targets, all antenna elements are involved to form the antenna beam through a change in the amplitude and phase control of these elements, resulting in a rather narrow antenna beam.
Such a solution is however inapplicable to a tracking antenna with a parabolic reflector of fixed dimensions. In such a case, the above-mentioned problems can be solved by making use of a reflector area of maximum dimensions for a tracking antenna with a suitably selected frequency to provide a rather narrow radar beam and, hence, less disturbance of the reflected radiation pattern. This is however possible only if during the elevation scan of the tracking antenna the probability of losing the target is minimized in the acquisition phase; therefore, it is imperative that the range and azimuth values of the target are continuously determined by the search radar as well as possible and that these values are employed optimally to align the tracking radar during acquisition. As concerns the determination of a highly accurate azimuth value of the target by the search radar, it is important to select the maximum dimensions of a search antenna.
Such a solution to the problem has however the disadvantage that the tracking antenna and the search antenna to be used are highly vulnerable due to the dimensions during bombardments, and during transport of the radar system, while operating, on a vehicle under extreme terrain conditions (e.g. a forested area).
According to the state of the art, another solution to the indicated problem of minimizing the probability of losing the designated target during acquisition is realizable with the tracking antenna performing a so called T.V. scan over a certain azimuthal sector in ascending direction. Such a solution however is successful only for slowly moving targets.